Introduction We investigated the mechanisms underlying reports of perceptual confidence. In particular, is all the information used for perceptual decisions available to confidence reporting? Some models of perception suggest that there are multiple channels of information, and subjective reports such as confidence ratings can only tap into one of the channels (e.g. cortical, but not subcortical channels). Is this intuitive view correct? We capitalize on an original psychophysical finding (Lau & Passingham 2006 PNAS) that subjective reports of perceptual confidence and perceptual performance (d') can dissociate, and apply formal model comparison techniques to identify the mechanism underlying confidence reporting.

Methods and Results We considered several signal detection theory models, including: A simple SDT model where confidence is determined by setting criteria on the primary decision axis; a late noise model where the noisy perceptual signal becomes even noisier when making confidence judgments; and a two-channel model where only one channel contributes to confidence judgments. We compared models by evaluating the likelihood of each model, given the metacontrast masking data, using the Akaike information criterion. All models could account for perceptual performance, but the late noise model provided the best fit to the observed performance-confidence dissociation.

Discussion Our results suggest that simple SDT models may not adequately characterize the relationship between perceptual decisions and confidence, because they do not provide a process that allows for the kind of performance-confidence dissociation observed in the metacontrast masking paradigm. However, this extra process need not be an extra information processing channel. Our best-fitting model was a hierarchical, single-channel model where noisy perceptual signals accrue further noise when used for rating confidence. This suggests that confidence decisions may be made by mechanisms downstream from perceptual decision mechanisms.